Assassin's Creed calls its technologically advanced precursor civilization the Isu.
The Isu are an ancient species that existed before recorded human history in the series' fictional universe. They created powerful artifacts known as Pieces of Eden and engineered humanity for labor and service. Their civilization was eventually devastated by a solar catastrophe.
Modern humans encounter the Isu mainly through archaeological discoveries, recovered memories, holographic messages, and artifacts. Important Isu figures include Minerva, Juno, and Jupiter, who appear in different games and storylines.
Earlier games often refer to the same civilization as the First Civilization or Those Who Came Before. Those are descriptive labels used within the franchise, but “Isu” became the standard specific name in later lore. The Isu are fictional and should not be confused with the historical Assassins or Templars, whose conflict forms the human side of the series.